1. Field of the Invention
This invention relates to a rotating pipe joint, which permits two substantially axially aligned joined pipes to rotate relative to one another. More particularly, this invention relates to providing a rotating pipe joint having a floating seal which provides an acceptable seal, for operation in a severe environment.
2. The Prior Art
The present invention was developed to meet the need in the air heater industry for an effective seal assembly for sealing the fluid conveying duct to the sootblower of a Rothemuhle air heater while permitting the two joined pipes to rotate relative to one another. The invention is naturally applicable wherever a rotating joint in a hostile environment is required and a less than complete seal is satisfactory.
In a Rothemuhle air heater, the heat transfer surfaces comprise a stationary stator, which is too large to be desirably rotatable. A hood above the stator and a hood below the stator rotate in unison, connected by a single main shaft having an axial hollow rotating centering post attached at each end. Aligned rotating portions of the upper and lower hoods provide one passageway for gas through the air heater surfaces. The remaining hood portions provide another passageway. Rotating hood portions may conduct flue gas or air, depending on the needs of a particular installation. The remaining hood portions conduct whichever fluid the rotating hood portion does not conduct. As the hoods rotate, heat transfer surfaces are alternately heated by flue gas and cooled by incoming combustion air.
The flue gas deposits deleterious materials, such as carbon, fly ash, etc., onto the heat transfer surfaces, reducing their ability to transfer heat. Efficient operation and long life require periodic removal of the build-up of deleterious materials from the heat transfer surfaces. Sootblowers have long been used for cleaning air heaters while they are in service. In a Rothemuhle air heater, the sootblower is attached to the upper or lower rotating hood, or in some cases a separate sootblower is attached to each rotating hood, so it naturally rotates in unison with the rotating hood. The orifices of the sootblower are aligned to blow high pressure fluid, customarily air or dry steam, into the heat transfer surfaces of the air heater, to dislodge deleterious material which has accumulated. Typically, a single sootblower is provided on the cold side of the air heater, however, a sootblower may also be provided on the hot side. The air heater hoods rotate in the range of about 3 to about 6 rpm.
The rotating centering posts, which link the upper and lower hood to the common main shaft, and which also serve as pipes for conveying fluid to the sootblower, must be connected to a stationary inlet pipe and the rotating joint between them must accommodate this rotation while providing an acceptable seal. The joint must survive in the hostile environment of the air heater where temperatures of 650.degree. F. or more are typically encountered. In addition, the rotating joint is inaccessible while the air heater is in service, and is difficult to reach in any event.
The prior art attempted to solve this problem by providing a sliding seal between the rotating centering post and the stationary inlet pipe. Generally, the sliding seal comprises a spring loaded joint having two hardened steel superposed rings, resiliently urged against one another by the springs, that rotatably slide relative to one another. The sliding seal, however, is susceptable to several difficulties. First, the area of the wear surfaces of the rings is relatively small and cannot be readily increased by redesign due to the overall configuration of the sliding seal. This leads unalterably to a relatively short service life; sliding seals often wear out after only three months of service and rarely last more than a year. Second, the sliding seal, despite the adjustability of the spring tension, cannot accommodate all the axial thermal growth of the main shaft and the rotating centering post, which may be as much as two inches. Typically, a standard expansion joint has been used on the end of the rotating centering post to accommodate much of this growth. In this usage, the standard expansion joint is subject to frequent failure. Third, the biasing springs which hold the rings in contact with each other must be preset to a certain tension unique to each air heater after installation in the field, to help properly accommodate the centering post and main shaft growth and provide sufficient pressure on the rings to make a good seal. This crucial adjustment requires skilled labor both to predict the required adjustment and to make the adjustment, but remains more art than science. This adjustment has a substantial effect on the life of the sliding seal but the accuracy of it is not known until failure of the seal. Fourth, the sliding seal cannot compensate for axial misalignment of the rotating entering post and the stationary inlet pipe. These two pipes are frequently misaligned or become so as the air heater wears. In this case, the rings of the sliding seal wear eccentrically instead of evenly, drastically shortening the life of the sliding seal, i.e., adequate pressure cannot be maintained within the sootblower system under normal operating conditions because one or both rings will wear completely through along some portion of its surface. If the system is being blown with steam, leaking steam can cut the pipe further, resulting in a complete loss of pressure.
The air heater is most conveniently removed from service for maintenance during regularly scheduled boiler maintainance, typically scheduled once a year. If the air heater must be removed from service while the boiler is operating, as is required if the sootblower fails because the sliding seal or the expansion joint fails, several undesirable effects occur. The air heater must be allowed to cool, which increases downtime. Valuable sensible heat of the flue gas is wasted since flue gas must bypass the air heater. Skilled workers must enter the air heater, discern the cause of the sootblower failure, and replace defective parts. The sliding seal may cost several thousand dollars, and the additional costs incurred in an unscheduled shutdown of the air heater make failure of the sliding seal especially expensive and undesirable.
Thus, a need exists for a rotating pipe joint which overcomes these and other difficulties associated with the prior art, which can be manufactured and sold at a reasonable price.